Microwave sources have been used to create plasmas adjacent a sample, such as a semiconductor substrate or other work piece, positioned within an evacuable processing chamber. During processing, the sample may typically either be etched or a thin film of material deposited thereon. In a typical plasma processing system, microwave energy is fed from a source, through a waveguide coupler, and into the evacuable processing chamber. The microwave energy interacts with a feed gas within the chamber to produce a plasma.
A particular type of microwave plasma processing system, the Electron Cyclotron Resonance (ECR) system, includes a magnetic field generator which produces a magnetic field that interacts with the feed gas excited by the microwave energy to create an electron cyclotron resonance condition of the electrons in the feed gas to thereby produce the plasma. An ECR system offers a number of advantages over other types of conventional plasma processing systems. For example, an ECR system provides electrodeless operation which reduces chamber wall contamination. The low plasma potential operation reduces ion induced surface damage to the sample. Moreover, high plasma density and low pressure operation can produce high deposition and etching rates with low particulate formation. These advantages have become increasingly important for processing state-of-the-art integrated circuits as device features are reduced to submicron dimensions.
The general design and operation of ECR systems in described in U.S. Pat. No. 4,859,908 to Yoshida et al. entitled Plasma Processing Apparatus for Large Area Ion Irradiation; U.S. Pat. No. 4,727,293 to Asmussen et al. entitled Plasma Generating Apparatus Using Magnets and Methods; U.S. Pat. No. 4,417,178 to Geller et al. entitled Process and Apparatus for Producing Highly Charged Large Ions and an Application Utilizing This Process; and Japanese published patent application 88-310887/44 entitled Film Forming Plasma-Generating Machine.
Present efforts to improve ECR systems have focused on increasing one or both of the efficiency of plasma generation, or the uniformity of the plasma generated. U.S. Pat. No. 5,003,152, to Matsuo et.al. for example, discloses a microwave coupler positioned between the source and the processing chamber that has a fixed dielectric slab mode transformer for converting an incident circular-waveguide TE.sub.11 mode into a hybrid TE/TM mode with an unspecified fraction of TM component. The hybrid mode contains both TE and TM components and, thus, can propagate only in the presence of the dielectric slab and plasma. The microwave coupler is directed toward improving the matching of the microwave energy to the plasma to increase the efficiency of plasma generation by more effectively coupling the microwave energy of the source to the plasma.
While efficiency is desirable so that a high density plasma can be generated using minimal microwave and magnetic power, uniformity is also a critical processing parameter to ensure uniform deposition or etching conditions across the entire sample surface. For example, as the size of semiconductor substrates increases, the need for uniformity in plasma processing becomes more critical. For example, U.S. Pat. No. 4,866,346 to Gaudreau et al. and entitled Microwave Plasma Generator discloses coupling a transverse magnetic mode with no angular dependence, such as the TM.sub.01 mode, from the microwave source to the plasma to thereby produce a circularly symmetric plasma. Unfortunately, the intensity of the microwave power flux produced is radially dependent and not uniform across the entire sample.
Another attempt at increasing uniformity by selecting the microwave source and its associated microwave coupler to produce a desired microwave energy is disclosed in U.S. Pat. No. 4,877,509 to Ogawa et al. entitled Semiconductor Wafer Treating Apparatus Utilizing a Plasma. The Ogawa et al. patent discloses a rectangular waveguide having a rectangular-to-circular converter and a circular polarization converter for transforming the circular TE.sub.11 mode from the source into a circularly polarized wave. Thus, the electric field strength of the microwave supplied into the evacuable chamber is azimuthally symmetric when averaged over time, to thereby create a more azimuthally uniform plasma density. However, the intensity of the microwave power flux is also radially nonuniform.
The magnetic field may also be shaped in an attempt to obtain greater overall uniformity of the plasma. For example, European Patent Application 87/311,451.6 to Nakamura et al entitled Plasma Apparatus describes an ECR system having a second magnet in addition to the primary magnet. The second magnet provides a more even distribution of the flux adjacent to the substrate being processed.
Despite continuing improvements in microwave plasma processing systems in general, and ECR systems in particular, there still exists a need for improvement in the radial uniformity of the plasma generated using a microwave source to produce a plasma in a processing chamber. This uniformity is critical as line widths and other semiconductor dimensions are decreased or as larger semiconductor substrates are used.